Method and arrangement for setting the remanent flux density in magnetic circuits and equalizer utilizing same

ABSTRACT

The remanent flux density in a selected portion of a magnetic circuit is precisely set to a desired value by comparing the flux density in the selected portion to a predetermined reference, generating an error signal based on the comparison, integrating the error signal, and inducing an H field in the magnetic circuit in accordance with the integrated error signal. Integration of the error signal is performed only when the flux density has a substantially remanent value, i.e., only when the H field in the magnetic circuit is substantially zero. Thus, error in setting the flux density due to imperfect remanence in the magnetic circuit is substantially eliminated. In an illustrative embodiment of the invention, the magnetic circuit comprises a magnetic core having an air gap, and is advantageously employed in a transmission cable equalizer. The remanent flux density in the air gap is utilized to precisely set the resistance value of a magnetoresistor, which in turn determines the gain of the equalizer.

United States Patent 1191 Ciesielka Nov. 13, 1973 METHOD AND ARRANGEMENT FOR SETTING TI-IE REMANENT FLUX DENSITY IN MAGNETIC CIRCUITS AND EQUALIZER UTILIZING SAME Primary Examiner-Paul L. Gensler Attorney-R. J. Guenther et al.

[57] ABSTRACT The remanent flux density in a selected portion of a magnetic circuit is precisely set to a desired value by comparing the flux density in the selected portion to a predetermined reference, generating an error signal based on the comparison, integrating the error signal, and inducing an H field in the magnetic circuit in accordance with the integrated error signal. Integration of the error signal is performed only when the flux density has a substantially remanent value, i.e., only when the H field in the magnetic circuit is substantially zero. Thus, error in setting the flux density due to imperfect remanence in the magnetic circuit is substantially eliminated.

I density in the air gap is utilized to precisely set the resistance value of a magnetoresistor, which in turn determines the gain of the equalizer.

15 Claims, 3 Drawing Figures [75] Inventor: Albert John Ciesielka, Atlantic Highlands, NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: Nov. 2, 1972 [21] Appl. No.: 303,137

[52] U.S. Cl 333/18, 307/101, 307/309, 317/123, 330/52, 330/132, 330/144, 333/16 [51] Int. Cl H04b 3/04, l-IOlf 13/00 [58] Field of Search 333/17, 18, 16; 307/101, 309; 317/123; 330/52, 132, 144

[56] References Cited UNITED STATES PATENTS 2,908,873 l0/l959 Bogert 333/18 3,257,628 6/1966 Kaenel 3,359,511 12/1967 Varnell, .lr.. 3,458,769 7/l969 Stampfli PILOT TONE SOURCE INFO L SOURCE 20 UTILIZATION CIRCUIT TONE DETECTOR RECTIFYING AND FILTERING PATENTEI] BUY I 3 I975 SHEET 1. CF 2 FIG.

UTILIZATION CIRCUIT PILOT *!5 TONE SOURCE INFO SOURCE PILOT TONE DETECTOR RECTIFYING AND FILTERING D. C. REFERENCE PAIENTEDRHV 3 ma SHEET 2 BF 2 7H FIELD FIG. 2A

1 (PULSES IN CORE 37a VOLTAGE AT TERMINAL R FIG. 2B-

T JWHTLHT in m N I. m m.

TIME

BACKGROUND OF THE INVENTION My invention relates generally to magnetic circuits and, more particularly, to a method and arrangement for precisely setting remanent flux density in magnetic circuits.

As herein defined, a magnetic circuit is an arrangement in which magnetic flux is produced in response to an applied magnetic field. A particular magnetic circuit may include one or more paths of magnetic material and a coil for applying the magnetic field thereto. The magnetic circuit may also include an air gap or gaps in one or more of the paths. The flux density in a particular path or portion of the circuit is determined principally by the circuit geometry, the strength of the applied magnetic field and the previous magnetic history of the circuit.

In the prior art, the flux density, or B field, in a selected portion of a magnetic circuit is generally set to a desired value by inducing a magnetic field, or H field, in the circuit such that the desired flux density is attained. Many magnetic materials, for example, ferromagnetic materials, exhibit substantial remanence, so that theI-I field in the magnetic circuit can be reduced to zero without the flux density being reduced to zero as well. Advantageously, then, a nonzero flux density can be permanently established in a selected portion of a magnetic circuit (for example, an air gap or a section of magnetic material). without expending energy to maintain same. I

However, remanence in magnetic materials is not perfect and the flux density does not remain precisely at a desired, originally set value. once the H field is removed. Rather, the flux density drops somewhat, retaining on the order of 80 to 90 percent of its original value. In certain applications, an error of this magnitude may be tolerable. In others, more precise flux density control is necessary.

One known arrangement for precisely setting flux density in a magnetic circuit maintains an H field in the circuit at all times. This arrangementprecludes advantageous utilization of magnetic remanence, as described above. Moreover, a failure in the H field supply will change the flux density from the desired value.

Another prior arrangement utilizes so-called square hysteresis loop materials which have remanences typically h..9 ...9 llFEi: Thes atsfi sar relatively expensive and thus not commercially viable for many applications.

Moreover, in certain magnetic circuit applications, an error of even several percent in a flux'density setting may be intolerable. For example, a magnetic circuit comprising a magnetic core having an air gap interrupting its continuity, can be utilized'in an arrangement'for controlling the gain of a transmission cable equalizer. A magnetoresistor is positioned in the air gap, its resistance value thus being determined by the remanent flux density in the gap. The equalizer gain is determined by the resistance value of the magnetoresistor. Thus, precise control of the equalizer gain requires that the remanent flux density in the air gap be precisely set.

SUMMARY OF THE INVENTION Accordingly, an object of the invention is to provide a method and arrangement for precisely setting remanent flux density in magnetic circuits.

A more specific object of the invention is to provide a method and arrangement for precisely setting the remanent flux density in an air gap in a magnetic circuit.

A further object of the invention is to provide a method and arrangement for precisely setting the flux density in the air gap of a magnetic core and thus the resistance value of a magnetoresistor positioned in the gap.

Another object of the invention is to provide a method and arrangement for precisely setting the flux density in the air gap of a magnetic core to precisely set the resistance value of a magnetoresistor to, in turn, precisely set the gain of a transmission cable equalizer.

These and other objects are achieved by comparing the flux density in a selected portion of a magnetic circuit to a predetermined reference, generating an error signal based on that comparison, integrating the error signal, and inducing an H field in the magnetic circuit in accordance with the integrated error signal to thereby change the flux density in the selection portion. In accordance with the invention, integration of the error signal is performed only when the flux density has substantially a remanent value; that is, only when substantially no H field is being induced in the magnetic circuit. In this way, the invention substantially eliminates error in setting the flux density due to imperfect magnetic remanence. In the prior art, as described above, a desired flux density is established while some H field is being induced in the magnetic circuit so that subsequent reduction of the H field to zero causes an unwanted change in the flux density.

An important feature of the invention is that the above-mentioned integrating and field inducing steps are performed independently in sequence; when the error signal is being integrated, no H field is induced in the magnetic circuit and when the H field is being induced in the magnetic circuit, no integration of the error signal is performed. The sequence of integrating and field inducing is performed repetitively until the flux density inthe selected portion of the magnetic circuit attains the desired value.

An illustrative embodiment of the invention precisely sets the flux density in the air gap of a magnetic core. The flux density is sensed by a magnetoresistor positioned in the gap, and the resistance value of the magnetoresistor establishes the amplitude of a control signal. The above-mentioned error signal is generated by determining the difference between the control signal and the above-mentioned predetermined reference.

In the illustrative embodiment, the magnetoresistor is advantageously utilized as a gain-determining element in a transmission cable-equalizer. A pilot frequency tone is transmitted down the cable to be equalized, and is thereby extended to the input of A an amplifier included in the equalizer. The pilot tone, detected at the equalizer output, is rectified and filtered to provide the above-mentioned control signal. A subtractor determines the difference between the control signal and a predetermined reference voltage proportional to that value of air gap flux density which provides a predetermined equalizer output amplitude. The resultant error signal is processed in accordance with the invention, as described above, to set the remanent flux density in the air gap, by which the resistance value of the magnetoresistor and hence the gain of the equalizer are set.

BRIEF DESCRIPTION OF THE DRAWING A clear understanding of the invention and of the preceding and other objects and features thereof may be gained from a consideration of the following detailed description and accompanying drawing in which:

FIG. 1 depicts an arrangement according to the invention, illustratively employed in a transmission cable equalizer; and

FIGS. 2A and 2B show graphs useful in describing the operation of the arrangement of FIG. 1.

DETAILED DESCRIPTION FIG. 1 depicts a magnetic circuit comprising a magnetic core 378, an air gap 379 interrupting the continuity thereof, and core windings 377. FIG. 1 further depicts circuitry in accordance with'the invention for precisely setting the remanent flux density in a selected portion of the magnetic circuit, viz. gap 379. The magnetic circuit is utilized illustratively in an automatic transmission cable equalizer 30 which includes an operational amplifier 31. As described in detail hereinafter, the magnitude of the flux density in gap 379 determines the gain of amplifier 31 and hence of equalizer 30 for a predetermined pilot frequency. Thus the present invention, by providing precise setting of the flux density in gap 379, assures precise setting of the gain of equalizer 30 at that pilot frequency.

The gain of equalizer 30 is a function of input frequency and is adjusted at each of (illustratively) four pilot frequencies f,, f f and f (which are distributed across the frequency band over which equalizer 30 is to operate) by adjusting the impedance of one of four equalizer input networks 34-37. For this purpose, networks 34-37 include variable resistances 34a-37a, respectively. Equalizer 30 is adapted for equalizing a transmission path comprising transmission cable 20 over which signals are extended from an information source to a utilization circuit. The frequencydependent gain characteristic of equalizer 30 is adjusted to correspond to frequency-dependent attenuation in cable 20. Thus, the frequency characteristic of signals from source 10 is substantially preserved at the input of the utilization circuit. When equalizer 30 is to be aligned, that is, when its gain characteristic is to be adjusted in accordance with the frequency-dependent attenuation in cable 20, tones at each of the abovementioned pilot frequencies are transmitted down cable 20 from a pilot tone'source 15. The impedances of networks 34-37 are adjusted by adjusting variable resistances 34a-37a, respectively such that the signal at output terminal 38 of amplifier 31 has a predetermined amplitude at each pilot frequency. Advantageously, this adjustment of variable resistances 34a-37a is effected automatically. For this purpose, connections between terminal 38 and each variable resistance are provided via taps off lead 33.

Each of the variable resistances 34a-37a comprises a magnetic core having an air gap, circuitry in accordance with the invention for precisely setting the flux density in the air gap, and a magnetoresistor positioned in the air gap. The magnetoresistor in each variable resistor comprises the resistive element thereof while the magnetic core and its associated circuitry control the resistance value of the magnetoresistor.

Thus, variable resistance 37a, for example, includes magnetic core 378, magnetoresistor MR positioned in gap 379, and circuitry for setting the flux density in gap 379. The latter comprises rectifying and filtering circuit 370, subtractor 371, error signal lead ER, switch 372, integrator 373, amplifier 374, switches 375 and 376 all connected in series in the order named. Switch 376 is connected to core windings 377. Variable resistance 37a also includes a pilot tone detector 380 which is coupled to output terminal 38 of equalizer 30, as is rectifying and filtering circuit 370. Detector 380 fcloses switch 376 when a tone at pilot frequency f is present at terminal 38.

Setting the flux density in gap 379 in accordance with the above-discussed prior art can be illustrated by assuming that when cable 20 is being equalized at pilot frequency f.,, switches 372 and 375 are both closed. Circuit 370 provides a dc. control signal proportional to the amplitude of the ac pilot signal (of frequency f4) at Output terminal 38. Subtractor 371 determines the difference between the control signal and a predetermined d.c. reference. The resultant error signal is extended along lead ER to integrator 373 and therefore to amplifier 374. The voltage at output terminal R of amplifier 374 generates a current in windings 377. The H field induced in windings 377 and thus in core 378 adjusts the flux density in gap 379, thus the resistance value of magnetoresistor MR and thus the gain of equalizer 30 at pilot frequency f.,.

The gain of amplifier 374 is chosen, based on the parameters of the other components of variable resistance 37a to ensure that the system is stable and that the error signal on lead ER will converge to zero. At that time, equalization of cable 20, i.e. alignment of equalizer 30 at pilot frequency f,, is complete. The signal at terminal R and, accordingly, the H field induced in core 378 become constant.

Thus, in the prior art, when the flux density in gap 379 and the resistance value of magnetoresistor MR are such that cable 20 is equalized at pilot frequency f,,

the H field in core 378 is not zero. If the H field in core 378 is thereafter reduced to zero to take advantage of the magnetic remanence of core 378, the flux density in gap 379 drops (assuming first quadrant operation) somewhat because the remanence in core 378 is not perfect. This, of course, lowers the resistance value of magnetoresistor MR and destroys the equalization. The remanence even of so-called square hysteresis loop materials, is generally too imperfect for precise equalization.

This problem in the prior art is alleviated in accordance with the present invention by integrating the error signal only when the flux density in the air gap has a remanent value; that is, only when the H field in the core is substantially zero. Thus, integration of the error signal on lead ER and induction of an H field in core 378 on the basis of the signal at terminal R, are performed independently in sequence in two distinct steps. During the first step, switch 372 is closed and switch 375 is opened so that a voltage proportional to the error signal on lead ER, and thus to the equalization error, is built up at terminal R with no current flowing in windings 377. During the second step, switch 372 is opened and switch 375 is closed so that current flows in windings 377 proportional to the equalization error that obtained when the flux density in gap 379 was at its last remanent value. The resultant l-I field induced in core 378 changes the flux density in gap 379, thereby adjusting the resistance value of ma'gnetoresistor MR and reducing the equalization error.

This two-step cycle of integration and induction, hereinafter referred to as a magnetization cycle, is performed repetitively until the desired flux density is achieved. Once the error signal on lead ER is zero at the start of a cycle, i.e. with switch 375 open, the flux density in gap 379 is assured to have the desired remanent value which provides alignment of equalizer 30 at pilot frequency )2. In the prior art, this same value of flux density would be attained with some nonzero H field induced in core 378. Thus it is seen that, advantageously, the present invention substantially eliminates equalization error due to imperfect remanence in core 378.

Although switches 372 and 375 are illustratively shown in FIG. 1 as mechanical elements, other known arrangements, for example, electronic switches, as'well as circuitry for operating them in the manner described above, will be apparent to those skilled in the art.

FIGS. 2A and 2B show graphs exemplifying the operation of equalizer 30 and, in particular, of variable resistance 37a, during alignment of the former at pilot frequency f,. FIG. 2A depicts variations in the gap 379 flux density in response to H field pulses induced in core 378 via windings 377. FIG. 2B depicts a time graph of those H pulses, as well as the signal at terminal R which generates them. The H scale along the ordinate of FIG. 2B is aligned with the H scale along the abscissa of FIG. 2A.

As seen from FIGS. 2A and 2B, the voltage at terminal R is initially zero and the flux density in gap 379 is B,. Assumptively, the flux density necessary to equalize cable 20 at pilot frequency f is B,,.

In the example of FIGS. 2A and 28, ten magnetization cycles, designated I, II...X, are needed to change the remanent flux density in gap 379 from B, to B The integration steps of cycles I, II...X are performed in time periods designated Ia, IIa...Xa, respectively. The induction steps are performed in time periods designated Ib, IIb,...Xb, respectively. During periods Ia, IIa,...Xa, switches 372 and switches 375 are closed and opened, respectively. During periods Ib, IIb,...Xb, the opposite relationship obtains.

Since B, is less than B,,. (the dc. reference provided to subtractor 371 corresponding to the latter), the error signal on lead ER is negative during period Ia and a positive ramp is generated at terminal R. In period Ib, then, positive current (as indicated by an arrow) flows into windings 377, inducing an H field pulse of magnitude H in core 378.

Although the rise time of the H field pulses in FIG. 2B are shown as substantially zero to simplify illustration, some nonzero rise time elapses before the pulses attain their full magnitude. It will be appreciated that this rise time is not an important consideration as long as periods Ib, Ilb...Xb have sufficient duration.

As seen in FIG. 2A, the flux density in gap 379 rises to point C in response to the H field pulse of period I'b. Since core 378 has imperfect remanence, the flux density in gap 379 falls back to point D when switch 375 opens at the start of period IIa. The flux density at point D is less than B so that the error signal on lead ER is still negative. Thus, the voltage at terminal R further increases during period IIa. In period IIb a pulse of magnitude H is induced in windings 377, driving the flux density in gap 379 to point E.

The flux density falls back to point P as period IIIa begins. The flux density is now greater than B so that the error signal on lead ER is positive and the voltage at terminal R decreases during period IIIa. The pulse induced during period III!) has magnitude H the flux density rising to point G but dropping back to point F as period IVa begins.

During period IVa, the terminal R voltage drops still further. The flux density rises to point I during period Nb and drops back to point F as period Vabegins.

In period Va the voltage at terminal R becomes negative and a negative H field pulse of magnitude H J is induced in windings 377 during period Vb. This reduces the flux density in gap 379 to point J. Due to imperfect remanence in core 378, the flux density rises to point K as period VIa begins. The flux density is reduced to point L during period VIbv As period VIIa begins, the flux density rises to point M at which point the flux density is again less than B, and the error signal on lead ER is again negative.

Accordingly, the voltage at terminal R increases during period VIIa. The flux density thus goes to point N in period VIIb, returns to and remains at point M during cycle VIII and rises to point 0 in cycle IX, from which it falls back to point P and finally rises to point Q in cycle X.

At the end of cycle X the flux density in gap 379 falls to B Thereafter, the error signal on lead ER is zero when switches 372 and 375' are closed and open, respectively, and accordingly, the voltage at terminal R stays constant. Thus, the flux density oscillates between points Q and B, during any magnetization cycles which follow cycle X, such as cycle XI. Generally, the number of magnetization cycles which will be needed to attain a particular remanent flux density, such as B,,, is not known beforehand, and some number of magnetization cycles corresponding to an anticipated maximum will occur during alignment at each pilot frequency.

The above described arrangement is merely illustrative of the invention. Thus, although use of the invention in conjunction with a particular magnetic circuit is disclosed herein, it will be appreciatedthat the invention can be utilized to set the remanent flux density in any magnetic circuit, whether or not it has an air gap, as long as the circuit has provision for detecting the flux density in the portion of the circuit of interest. An example of such a magnetic circuit is the transfluxor, described for example by J. A. Rajchman and A. W. L0 in Proceedings of the IRE, Vol. 44, page 321, March It is to be understood therefore that numerous and other varied arrangements in accordance with the principles of the invention may be devised by those skilled in the art without departing from the spirit and scope thereof.

I claim:

1. A method for setting the flux density in a selected portion of a magnetic circuit comprising the steps of,

generating an error signal corresponding to the difference between the flux density in said selected portion and a desired value therefor,

integrating said error signal, and

inducing an H field in said circuit in accordance with the integrated error signal,

characterized in that said integrating step is performed only when the flux density in said selected portion has substantially a remanent value.

2. A method in accordance with claim 1 wherein said integrating and inducing steps are performed independently in sequence.

3. A method in accordance with claim 2 wherein said generating step comprises the steps of,

sensing the flux density in said selected portion,

providing a control signal having an amplitude proportional to the sensed flux density,

providing a reference signal corresponding to said desired flux density value, and

determining the difference between said control signal and said reference signal to generate said error signal.

4. A method in accordance with claim 3 wherein said selected portion is an air gap and wherein said sensing step is performed using a magnetoresistor positioned in said air gap.

5. A method in accordance with claim 4 wherein said control signal providing step comprises the steps of extending a predetermined signal to the input of an amplifier and adjusting the gain of said amplifier based on the resistance value of said magnetoresistor, said control signal being derived from the output signal of said amplifier.

6. An arrangement forsetting the flux density in a selected portion of a magnetic circuit comprising, means for generating an error signal corresponding to the difference between the flux density in said selected portion and a desired value therefor, means operative for integrating said error signal, and means operative for inducing an H field in said magnetic circuit in accordance with the integrated error signal, the improvement comprising means for operating said integrating means only when the flux density in said selected portion has substantially a remanent value.

7. An arrangement in accordance with claim 6 wherein said last-mentioned means comprises means for operating said integrating means and said inducing 9. An arrangement in accordance with claim 8 wherein said selected portion is an air gap and wherein said sensing means includes a magnetoresistor positioned in said air gap.

10. An arrangement in accordance with claim 9 wherein said control signal providing means comprises an amplifier, means for extending a predetermined signal to the input of said amplifier and means for adjusting the gain of said amplifier in accordance with the resistance value of said magnetoresistor, said control signal being derived from the output of said amplifier.

11. In an equalizer including circuit means adapted for insertion into a transmission path and operative for varying the gain of said path the combination comprising, a magnetic core having an air gap interrupting its continuity and providing remanent magnetic flux in said air gap, means for operating said circuit means in accordance with the density of magnetic flux in said air gap, means coupled to said circuit means fordetecting a pilot signal applied to said path, means for comparing the detected pilot signal to a predetermined reference signal and for generating an error signal based on the comparison, means operative for integrating said error signal only when the flux density in said gap has substantially a remanent value, and means operative for applying a magnetic field to said core to adjust the density of the flux in said air gap, said magnetic field being proportional to the output of said integrating means.

12. An equalizer in accordance with claim 11 wherein said last-mentioned means comprises means for operating said integrating means and said applying means independently in sequence.

13. An equalizer in accordance with claim 12 wherein said means for operating said circuit means includes magnetoresistive means positioned in said air gap.

14. An equalizer in accordance with claim 13 wherein said detecting means includes means for providing a dc control signal proportional to said detected pilot signal and where said comparing means includes means for determining the difference between said control signal and said error signal.

15. An equalizer in accordance with claim 14 further comprising means for amplifying the output of said integrating means, said magnetic field being proportional to the output of said amplifying means. 

1. A method for setting the flux density in a selected portion of a magnetic circuit comprising the steps of, generating an error signal corresponding to the difference between the flux density in said selected portion and a desired value therefor, integrating said error signal, and inducing an H field in said circuit in accordance with the integrated error signal, characterized in that said integrating step is performed only when the flux density in said selected portion has substantially a remanent value.
 2. A method in accordance with claim 1 wherein said integrating and inducing steps are performed independently in sequence.
 3. A method in accordance with claim 2 wherein said generating step comprises the steps of, sensing the flux density in said selected portion, providing a control signal having an amplitude proportional to the sensed flux density, providing a reference signal corresponding to said desired flux density value, and determining the difference between said control signal and said reference signal to generate said error signal.
 4. A method in accordance with claim 3 wherein said selected portion is an air gap and wherein said sensing step is performed using a magnetoresistor positioned in said air gap.
 5. A method in accordance with claim 4 wherein said control signal providing step comprises the steps of extending a predetermined signal to the input of an amplifier and adjusting tHe gain of said amplifier based on the resistance value of said magnetoresistor, said control signal being derived from the output signal of said amplifier.
 6. An arrangement for setting the flux density in a selected portion of a magnetic circuit comprising, means for generating an error signal corresponding to the difference between the flux density in said selected portion and a desired value therefor, means operative for integrating said error signal, and means operative for inducing an H field in said magnetic circuit in accordance with the integrated error signal, the improvement comprising means for operating said integrating means only when the flux density in said selected portion has substantially a remanent value.
 7. An arrangement in accordance with claim 6 wherein said last-mentioned means comprises means for operating said integrating means and said inducing means independently in sequence.
 8. An arrangement in accordance with claim 7 wherein said generating means comprises means for sensing the flux density in said selected portion, means for providing a control signal having an amplitude proportional to the sensed flux density, means for providing a reference signal corresponding to said desired flux density value, and means for determining the difference between said control signal and said reference signal to generate said error signal.
 9. An arrangement in accordance with claim 8 wherein said selected portion is an air gap and wherein said sensing means includes a magnetoresistor positioned in said air gap.
 10. An arrangement in accordance with claim 9 wherein said control signal providing means comprises an amplifier, means for extending a predetermined signal to the input of said amplifier and means for adjusting the gain of said amplifier in accordance with the resistance value of said magnetoresistor, said control signal being derived from the output of said amplifier.
 11. In an equalizer including circuit means adapted for insertion into a transmission path and operative for varying the gain of said path the combination comprising, a magnetic core having an air gap interrupting its continuity and providing remanent magnetic flux in said air gap, means for operating said circuit means in accordance with the density of magnetic flux in said air gap, means coupled to said circuit means for detecting a pilot signal applied to said path, means for comparing the detected pilot signal to a predetermined reference signal and for generating an error signal based on the comparison, means operative for integrating said error signal only when the flux density in said gap has substantially a remanent value, and means operative for applying a magnetic field to said core to adjust the density of the flux in said air gap, said magnetic field being proportional to the output of said integrating means.
 12. An equalizer in accordance with claim 11 wherein said last-mentioned means comprises means for operating said integrating means and said applying means independently in sequence.
 13. An equalizer in accordance with claim 12 wherein said means for operating said circuit means includes magnetoresistive means positioned in said air gap.
 14. An equalizer in accordance with claim 13 wherein said detecting means includes means for providing a d.c. control signal proportional to said detected pilot signal and where said comparing means includes means for determining the difference between said control signal and said error signal.
 15. An equalizer in accordance with claim 14 further comprising means for amplifying the output of said integrating means, said magnetic field being proportional to the output of said amplifying means. 